A conventional plunge router generally has a motor housing assembly translatably mounted upon a pair of laterally spaced columns which are affixed to an annular base. The annular base has a planar work engaging surface to which the columns are perpendicularly orientated. The motor housing assembly carries a cutting tool and may be moved axially relative to the annular base so that the cutting tool may engage and cut a work surface on a workpiece.
A pair of bushings are typically supported within a housing of the motor housing assembly. The bushings provide lateral support to and slide upon the spaced columns to accommodate translational or axial movement between the motor housing assembly and the annular base.
In order for the cutting tool to accurately cut the workpiece, it is important that the axis of the cutting tool, which is parallel to the columns, be maintained perpendicular to the planar work engaging surface of the base. Therefore, tolerances between the inner diameters of the bushings and the outer diameters of the columns, as well as between the bushings and the housing, must be held fairly closely to maintain the perpendicular relationship between the axis of the cutting tool and the planar work engaging surface of the annular base.
However, providing too tight of tolerances between the bushings supported by the housing and the columns can present problems. First, certain minimum tolerances are needed to accommodate manufacturing considerations such as the dimensional stack-ups of components which are joined together. Second, tolerancing must be provided to accommodate thermal expansion and contraction differences between the annular base, to which the columns are affixed, and the housing, which supports the bushings which, in turn, provide lateral support to the free ends of the columns. If this difference becomes large, then the columns will no longer be held parallel. Consequently, the bushings may bind when sliding upon the columns if there is insufficient tolerancing or spacing between the housing, the bushings and the columns.
Conventionally, both the base and the portion of the housing supporting the bushings are made of the same material so that they will have identical thermal coefficients of expansion. Consequently, the base and the bushing supporting portion of the housing expand or contract nearly identically and the columns remain parallel to one another. Due to strength and weight consideration, die cast aluminum is a preferred material for the base and the portion of the housing supporting the bearings.
However, making the housing entirely or partially out of die cast aluminum is costly and the strength of the aluminum is not needed in the housing. Accordingly, it would be advantageous to make a housing out a cheaper material, such as plastic, while still using a metal or die cast aluminum base.
The present invention is intended to overcome the above-described shortcomings of plunge routers using conventional mounting assemblies to join motor housing assemblies to columns and their bases.